Well you haven't been addressing core stabibility. That's a simple place to start. Sent, Pity...these guys no what I'm talking about and can hook you up and I'm sure they'll do that (right guys?
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I wouldn't go right to GM's for "lower back" I'd start with some simple back extensions. Concentrating of course on hip extension (using the glutes) and not lumbar extension. With lumbar it's about stabibility and KEEPING your body from moving back and forth. Doing some extensions really nice and controlled with a nice squeeze at the top, back NOT hyperextended of course, and chest puffed forward shoulders back, etc. will do you good. Add to that some time STATIC extensions.
And THAT is the biggest function of the core....not to make you go back and forth like with flexion movements and the like...it is to KEEP you from doing that. So core STABIBILITY and core STRENGTH are not necessarily the same thing. With stabibility it's about endurance and this is involving strong isometric contractions. So think about that when you're body is moving in a direction you don't want it to.
Damn, it's getting late. I'll look in on you later...